Skype users running lie-detecting software, promoted by the company , must ensure they inform other parties on the call or risk breaking a range of privacy acts.

Kishkish is lie-detector software that works by analysing the stress levels in a caller's voice and giving the user an indication in real-time of whether or not it thinks they are telling a lie. It can also be used with another program, Kishkish SAM VSA to record the call and analyse the stress level at a later time.

However, users have been warned that they must take care when using the software.

Although it is not illegal for the device to be used in respect of a private telephone conversation, Stephen Sidkin, partner at Fox Williams LLP solicitors, said: “If the other party to the conversation was not informed of the recording it is likely that the passing of the recording to a third party would result in an infringement of the Regulation of Investigatory Powers Act 2000 (RIPA).”

RIPA is a UK law which covers the interception of communications and was introduced to take account of technological change such as the growth of the internet and strong encryption. Those who break it could receive up to two years in jail or a fine of up to £5,000.

The software is compatible with Skype 3 and works by using the first 10 seconds to decipher the general stress level of the speaker before giving users four indicators about the stress level of the speaker.

This is either done via a needle that moves up and down and registers the stress level of the speaker or through a meter which measures the stress level from 0 to 100.

There is also a green light that changes to red when stress levels are above normal and finally, on top of the display there is a message which indicates the stress level of the speaker from high to normal.